(a) Field of the Invention
The present invention relates to valves and, in particular, to an improved hermetically sealed diaphragm globe valve for use with hazardous, toxic or radioactive fluids where escape to atmosphere could cause fire, explosion or be poisonous.
(b) Description of the Prior Art
Maximum reliability over a life-span of 40 years is a basic requirement for valves in nuclear service. This emphasis on long-term reliability arises as a result of the need to ensure that nuclear plants do not threaten public safety due to malfunction or leakage of radioactive coolant. In addition, corrective maintenance under radiological control procedures is very difficult and generally not anticipated.
There are basically two potential leakages in standard valves. For ease of maintenance, gasketed bolted bonnet designs are widely used with provision for seal welding. If seal welding is not used, the reliability of the gasket joint is questionable because of relaxation of gasket materials and body bonnet bolting due to thermal fatigue. Seal welding can eliminate the body bonnet joint leakage; however, removal of the weld is required in order to service the internal components of the valve. To eliminate potential leakage of body bonnet joints, welded bonnet or bonnetless designs are available. The second potential leakage, which is through the stem seal, cannot be eliminated in these designs.
In an attempt to eliminate entirely the potential leakage through body-bonnet joints and stem seals, bellows seal valves have been utilized, these valves offering a complete solution to the potential leakage. The disadvantage associated with bellows seal valves is the considerable length of the bellows which, depending on the operating pressure, must be 8 to 12 times the valve lift. This makes the overall length of a valve with a bellows seal, especially on large gate valves, impractical. In addition, it is difficult to predict the quality and reliability of the metal bellows required for high pressure service, although considerable progress has been made in the last few years and reasonably reliable solutions have been found.
In recent years, metal diaphragm valves for globe valves in sizes 1/4-2" have been developed to compete with bellows seal valves. This was possible due to the availabilty of a highly efficient nickel-chromium alloy type Inconel 718 for the construction of the metal diaphragm. The diaphragm consists of a series of specially shaped plates separating the valve disc and stem and providing simultaneously the seal for the body-bonnet joint and stem. Since the left in such valves is limited, the valve body geometry must be inclined to provide reasonable flow conditions with minimum resistance.
In one known metal diaphragm valve design, the metal diaphragm consists of a multiple set of thin plates of Inconel 718, clamped between the body and bonnet of the valve, establishing a metal-to-metal seal. The large compressive forces required to seal the peripheral edge of the diaphragm are provided by a long, threaded engagement between body and bonnet. A stem and valve disc are separated by the diaphragm.
There is, however, little difference between this nuclear valve design and a standard low-priced "union bonnet" globe valve used in low pressure applications in refineries with a metal-to-metal body-bonnet joint. Further, the valve construction is not considered to be hermetically sealed, but rather a metal gasketed valve with a screwed bonnet. Leakages to the environment can occur under the following circumstances even with the diaphragm itself in operating condition. The screwed bonnet can vibrate off or the original torque required for the seal can be reduced by thermal fatigue. This would relax the metal-to-metal contact pressure, resulting in leakage around the diaphragm through the threaded portion to the atmosphere, as well as around the stem, unless the secondary stem seal with standard packing rings is tight.
The multi-ply metal diaphragm acts as a metal gasket which, in itself, is not considered as an efficient gasket type for high pressure-temperature applications. As well, screwed joints without seal welding are not recommended by the ASME Section III Code. However, with this particular design, even a seal weld between body and bonnet would eliminate only the possibility of leakage through the body-bonnet joint, with the leak pattern around the stem still existing, thus exposing the secondary stem seal to full pressure. Also, a seal weld will eliminate the possibility of readjustment.
In an attempt to eliminate the disadvantages associated with the metal diaphragm valve referred to above, it has been proposed to place the metal diaphragm plates in a cartridge unit which is then placed in the body of the valve without welding. Tightness between diaphragm and body is achieved by a pressure seal effect. The bonnet is screwed into the body and seal welded to prevent leakage to the environment through the body-bonnet joint. While the leak pattern through the body bonnet joint is eliminated in this design, the leak pattern around the cartridge-body seal and around the stem seal is still possible even with a perfectly intact diaphragm.
To eliminate entirely the potential leakage through body-bonnet joints and stem seals U.S. Pat. No. 3,874,636, issued Apr. 1, 1975 to Bake, et al discloses a hermetically sealed valve intended to prevent environmental leakage and includes a multi-ply flexible metal diaphragm which must be seal welded directly to the bonnet, as well as requiring a seal welded body-bonnet joint; this construction is intended to eliminate the possibility of leakage through a mechanical joint used in the above-noted diaphragm valve construction.
A problem associated with the Bake, et al valve construction is the fact that the multi-ply metal diaphragm must be welded directly to the bonnet; in particular, the thin metal discs which form the metal diaphragm can become distorted during welding thereof to the heavy bonnet and to a metal ring situated therebeneath; the thin metal discs forming the diaphragm are made from Inconel 718 nickel chromium alloy, which is a high strength, corrosion resistant material having a very high fatigue strength; this latter feature, which is of importance insofar as the operation of the metal diaphragm is concerned, can only be achieved if the Inconel 718, after forming and welding, is annealed and aged. In the annealed condition, the Inconel 718 has a yield strength of 76,000 PSI at room temperature; after aging, the yield strength is increased to 150,000 PSI, and the high temperature fatigue strength at 600.degree. operating temperature after B 10.sup.5 cycles is 115,000 PSI. The annealing temperature is 1750.degree. F. for one hour with subsequent air cooling. The most suitable aging recommended for the service in a diaphragm valve by INCO is 1325.degree. F. for 8 hours, furnace cooled to 1150.degree. F. and aging time of B 4 hours with subsequent air cooling.
In the case of nuclear diaphragm valves such as that disclosed by Bake, et al, the body and bonnet are generally produced from austenitic stainless steel, or the like. After welding the Inconel metal discs, which form the metal diaphragm to the bonnet, problems can be encountered insofar as heat treating the diaphragm plates and the weld. In particular, the heat-treating requires temperatures in excess 1300.degree. F. to eliminate stresses in the Inconel plates created during the forming and welding of the metal plates. The bonnet, being manufactured from austenitic stainless steel, cannot be subjected to such temperatures in excess of 800.degree. F. due to carbine precipitation which would render the stainless steel unusable without subsequent solution annealing at 2100.degree. F., then quenching in water. However, solution annealing is not acceptable from the standpoint of the high tensile properties of the Inconel 718 acheieved during the aging process, which would be destroyed by solution annealing. Also, solution annealing could develop cracks in the seal weld. It is, therefore, difficult with the Bake, et al valve design to achieve optimum design conditions for Inconel 718 recommended by the International Nickel Company in their catalogue entitles "Inconel Alloy 718", Second Edition 1973.
Since the metal diaphragm discs or plates cannot readily be subjected to proper annealing and aging the cycle life of the metal diaphragm could possibly be limited. Further, in view of the thinness of the metal plates or discs forming the diaphragm, the diaphragm can become distorted during welding thereof to the heavy bonnet. Since the discs must be welded carefully to the bonnet and ring, replacement of the diaphragm cannot be readily effected at the site, but rather must be carried out in a factory with special welding equipment under special control conditions. As a result, unnecessary down time to replace a broken diaphragm can result when utilizing a metal diaphragm welded directly to the bonnet and ring components of the prior art diaphragm valve.